1. Field of the Invention
The present invention relates to cooling a scientific instrument using cryogenic liquids. More specifically, the present invention relates to a heater control system that generates a coolant gas by boiling a cryogenic liquid to cool a sample disposed in a thermal chamber of an instrument.
2. Related Art
Scientific instruments are often used to study the properties of materials at low temperatures. One method for achieving the low temperatures is to evaporate a coolant gas from cryogenic fluid. This is accomplished using heaters immersed in the cryogenic fluid. The coolant gas is subsequently injected into the instrument to achieve the desired low temperatures. The amount of cooling is determined by the amount of cryogenic fluid evaporated to produce the coolant gas. The greater the amount of heat generated by the heaters, the greater the among of coolant gas produced, and correspondingly, the greater the cooling effect. The amount of heat generated by the heaters is determined by the amount of alternating electrical current supplied to the heaters.
It is well known in the art to use a triode AC switch (triac) to control the supply of alternating electrical current to heaters, lights, and other such devices. A triac produces current for a fraction of the time according to the application pulses to a triac activation gate. It can be used in two different modes, phase angle firing and time proportioning. In the phase angle firing mode, the triac activation gate pulses are controlled so that the triac conducts during a portion of each AC half cycle and is left off for the remainder of each half cycle. The current delivered is a function of the fraction of the cycle that the triac conducts. In the time proportioning made, the triac is switched on for an entire cycle. However, the triac is only switched on a fraction of the total number of available power cycles. The current delivered is proportional to the fraction of the available power cycles that the triac conducts.
There are significant problems associated with switching large current, such as those required for heaters. One problem is the disturbance to nearby electronic circuits caused by the switching. For example, lights may flicker as the large currents are switched in and out. Moreover, phase angle firing creates harmonics on an AC power supply which can cause noise bursts that adversely affect other electronic equipment on the same supply. Because of these undesirable effects, regulatory agencies such as the International Electrotechnical Commission (IEC) have imposed limits on how rapidly the loads may be controlled using switching devices such as triacs. However, because triacs are simple, inexpensive and reliable devices, they are widely used.
Thus, there is a need for a controller that can control switching of load in an IEC compliant manner, while delivering sufficient power that is required in more power intensive applications such as heater controllers. For example, such a controller is desirable to provide current to heaters in thermal analysis instrumentation that provide desired cooling environments for low temperature studies.